1. Field of the Invention
The invention relates to the technology of estimating the upper limit of the state of charge of a storage device that is installed on a vehicle and can be externally charged (i.e., charged with an external power supply). In particular, the invention is concerned with the technology of estimating the upper limit, based on the states of charge of the storage device before and after the external charging.
2. Description of the Related Art
A secondary battery that is installed on a vehicle including a dynamo-electric machine or machines as a power source and supplies electric power to the dynamo-electric machine(s) is repeatedly charged and discharged; therefore, the upper limit of the state of charge (which will also be referred to as “full-charge capacity”) of the secondary battery decreases with time due to its chronological changes. As the full-charge capacity decreases, the distance over which the vehicle can travel with the dynamo-electric machine(s) is reduced. Therefore, it is necessary to detect a reduction of the full-charge capacity with high accuracy so as to accurately grasp the distance over which the vehicle can travel with the dynamo-electric machine(s).
Japanese Patent Application Publication No. 2003-068369 (JP-A-2003-068369) discloses a method of detecting the total capacity of a secondary battery, with the objective of easily detecting the total capacity of the secondary battery. According to the method disclosed in the above-identified publication, the total capacity of the secondary battery is detected based on an integrated current value obtained by integrating charging/discharging current over a period of transition from a first state in which no charging/discharging current flows in the secondary battery for a given time or longer to a second state in which no charging/discharging current flows in the secondary battery again for a given time or longer, and a difference between the charge level in the first state and the charge level in the second state. The method is characterized in that voltages of the secondary battery in the first state and the second state are measured, and each of the charge levels is determined based on a preliminarily acquired relationship between the voltage of the secondary battery and the charge level.
According to the method disclosed in the above-identified publication, the total capacity of the secondary battery can be easily detected.
However, since the state of charge of the secondary battery varies largely due to charging or discharging during running of the vehicle, the reduction of the full-charge capacity of the secondary battery may not be detected with high accuracy. In the above-described method of detecting the total capacity of the secondary battery, this problem is not taken into consideration.